Influenza is caused by an RNA virus of the orthomyxoviridae family. There are three types of these viruses and they cause three different types of influenza: type A, B and C. Influenza virus type A viruses infect mammals (humans, pigs, ferrets, horses) and birds. This is very important to mankind, as this is the type of virus that has caused worldwide pandemics. Influenza virus type B (also known simply as influenza B) infects only humans. It occasionally causes local outbreaks of flu. Influenza type C viruses also infect only humans. They infect most people when they are young and rarely causes serious illness.
Current rapid immunodiagnostic tests for influenza antigens like “BINAX NOW FLU A and FLU B™” (Binax, Inc., Portland, Me), “DIRECTIGEN FLU A+B™” (Becton Dickinson, Franklin Lakes, N.J.), “FLU OIA™” (Biostar Inc., Boulder, Colo.), “QUICK VUE™” (Quidel, San Diego, Calif.), “INFLU AB QUICK©” (Denka Sieken Co., Ltd., Japan) and “XPECT FLU A & B” (Remel Inc., Lenexa, Kans.), can reportedly either detect influenza A or distinguish between influenza A and B. The complexity of the test formats may require special training. In addition, significant amounts of virion particles are commonly required to obtain a positive test result, limiting their use to a short window of time when virus shedding is at its highest levels. Assay sensitivity is also variable with up to 20% false negative test results in certain assays being of significant current concern (e.g., see “WHO recommendations on the use of rapid testing for influenza diagnosis,” July 2005). Reverse-transcriptase PCR-based diagnostics (RT-PCR) has resulted in advances in capabilities, but is laborious and requires highly trained personnel making on-site or field-testing difficult. Because of the relative inefficiency of the reverse transcriptase enzyme, significant amounts of virus (e.g., 104 virion particles) and as many as 20 primers may be required effectively to detect viral RNA. Unfortunately, RT PCR is not easily adapted to high throughput screening of subjects in an epidemic setting or to field uses in an agricultural or point-of-care setting.
Additionally, the complexity, diversity and rapid emergence of new influenza strains has made diagnosis of high risk strains difficult, and therefore rapid response is at present nearly impossible. For epidemiologists, diversity resulting from high mutation rates and genetic reassortment make it difficult to anticipate where new strains may originate and respond with the timely introduction of new diagnostic primers for PCR. As a result, (at present) the diversity of influenza dictates the necessity of multiplex PCR approaches.